1. Field of the Invention
The present invention relates to an image forming apparatus for writing an image in synchronization with a main scanning operation using a laser beam. The present invention also relates to an apparatus for forming an image by correcting a time lag at a writing time of the image. Further, the present invention relates to an image forming apparatus constructed such that no shift in position of images is caused when overlapping and transferring operations with respect to the images are performed.
2. Description of the Related Art
In a normal analog copying machine, an encoder plate is attached to a drive shaft for a photosensitive body and a basic pulse is formed by the encoder plate in accordance with the rotation of the drive shaft for the photosensitive body. This basic pulse is used in a control section to count up a counter having a timer function, thereby performing sequential control of the copying machine.
Such a controller is disclosed in e.g., Japanese Patent Application Laying Open (KOKAI) Nos. 62-242471 and 63-70265. When the above sequential control method of the analog copying machine is applied to a digital copying machine for forming an electrostatic latent image by performing a scanning operation using a laser beam, the following problems are caused.
In the sequential control applied to the above analog copying machine, no used basic pulse is synchronized with a main scanning line of the laser beam. Accordingly, there is a case in which the timing of a writing operation of the image is shifted by one line at its maximum, thereby causing an error in writing operation of the image.
Therefore, when this sequential control system is applied to the digital copying machine for forming an electrostatic latent image by performing a scanning operation using the laser beam, no problems are almost caused in the case of a black-and-white copying machine. However, the above-mentioned error in writing operation of the image causes a shift in color in the case of a color copying machine so that the quality of the image is greatly reduced in a certain case.
It is considered to prevent the shift in color by turning on a gate for starting the writing operation for forming the image by using a detecting signal of the synchronization of the laser beam on the main scanning line. However, in this method, the accuracy in constant rotation of the photosensitive body is reduced so that the shift in color cannot be prevented when the rotation of the photosensitive body is unstable.
To solve this problem, a controller for a copying machine is proposed in Japanese Patent Application Laying Open (KOKAI) No. 63-70265. In this controller, start timings of the photosensitive body and an optical scanning mechanism are synchronized with each other to align images with each other at a transfer time.
However, the structure of such a controller is complicated and manufacturing cost is high.